No government has publicly stated that it is building autonomous weapons, but there are several reasons why they might start. The need for speed has already led at least 30 countries to deploy defensive systems with human-supervised autonomous modes, such as Aegis and Patriot, to protect ships, bases, and civilian populations from swift swarms of aircraft and missiles. Such systems are only likely to become more important as precision-guided missiles proliferate. Autonomous weapons could also be useful in situations where radio links work badly or not at all. In a conflict, militaries will seek to jam or disrupt each other’s communications. Moreover, some environments, such as undersea, are intrinsically challenging for communications. Finally, some governments could desire autonomous weapons, in part, simply because they believe potential adversaries might obtain them.